Conventionally, in an electrophotographic image forming apparatus and others, a latent image which is electrically or magnetically formed is made apparent by an electrophotographic toner (hereinafter, simply referred to as “toner”). For example, in an electrophotographic method, an electrostatic image (latent image) is formed on a photoconductor, and the latent image is then developed by toners to form a toner image. The toner image is usually transferred on a transfer material such as paper and then fixed on the transfer material such as paper. In a fixing step during which the toner image is fixed on transfer paper, heat fixing methods such as a heating roller fixing method and a heating belt fixing method have been widely used due to an excellent energy efficiency thereof.
In recent years, there have been increasing demands from the market for higher speeds and greater energy saving of image forming apparatuses. Accordingly, a toner excellent in lower-temperature fixing property and also capable of providing high quality images is demanded. In order to realize lower-temperature fixing property of the toner, it is necessary to lower a softening temperature of the binding resin of the toner. However, if the binding resin is low in softening temperature of the binding resin, so-called offset (hereinafter, also referred to as “hot offset”) may easily occur in which a toner image partially adheres to the surface of a fixing member when fixing and the thus adhered image is then transferred on copier paper. Further, so-called blocking will take place in which the toner is lowered in heat-resistant storage stability and toner particles are fused to each other particularly in a high temperature environment. In addition, there have been found such problems that in a developing device, a toner is fused inside the developing device and a carrier to contaminate and toner filming easily occurs on the surface of a photoconductor.
Such technologies for solving the above problems are known such as using a crystalline resin as a binding resin of a toner. That is, the crystalline resin is able to soften rapidly at a melting point of resin and therefore able to lower a softening temperature of toner close to the melting point, while securing the heat-resistant storage stability at a temperature lower than the melting point. Therefore, it is possible to attain the heat-resistant storage stability and lower-temperature fixing property at the same time.
As a toner which uses a crystalline resin, there is disclosed, for example, a toner using as a binding resin a crystalline resin which is prepared by elongating crystalline polyester with diisocyanate (refer to PTLs 1 and 2).
Further, such a toner is proposed that uses a crystalline resin with a crosslinked structure by unsaturated bonding containing a sulfonic group (refer to PTL 3). This toner has been improved in hot offset resistance as compared with conventional arts. There is also disclosed a technology in which a ratio of softening temperature to peak temperature of fusion heat and viscoelastic characteristics are specified to produce resin particles which are excellent in lower-temperature fixing property and heat-resistant storage stability (refer to PTL 4).
There is also disclosed a technology in which a crystalline resin is specified for durometer hardness and inorganic fine particles are contained into a toner to improve stress resistance of the toner (refer to PTL 5).
On the other hand, unlike the above-described known technologies in which a crystalline resin is used as a major composition of a binding resin, there are disclosed many technologies in which a crystalline resin and a non-crystalline resin are used in combination (for example, refer to PTLs 6 and 7).
However, a pigment contained in a toner may be unevenly distributed on the surface of the toner or may produce a large aggregate due to compatibility with a material used. Therefore, for example, as disclosed in PTL 8, such a method is commonly employed that a pigment dispersing agent is used to uniformly disperse the pigment inside the toner.
However, many of the pigment dispersing agents are non-crystalline. Where a crystalline resin is contained and in particular where the crystalline resin is used as a main binder, compatibility is poor, thus resulting in a situation that a pigment and a dispersing agent thereof produce a large aggregate or are unevenly distributed on the surface of the toner. As a result, an effect that the pigment is uniformly dispersed inside the toner is not obtained but the pigment on the surface of the toner adversely influences the charging property of the toner. Thus, defects occur in a machine when developing or transferring, which causes poor images such as blushing.
As described above, where a crystalline resin is used as a binding resin of a toner, even if fixing temperature, heat-resistant storage stability and stress resistance can be improved, the state of the pigment contained therein cannot be favorably improved. As a result, the toner is insufficient in quality for use.
Further, where a crystalline resin is used as a binding resin, such a problem is posed that a pigment is lowered in dispersion property to result in reduced image density. For example, there is proposed such a toner with base particles that is produced through a step in which, for example, a binding resin containing at least polyester soluble in an organic solvent as a major composition, a colorant master batch containing a colorant and a coloring-agent dispersing resin, and a toner composition liquid in which a mold releasing agent is dissolved or dispersed in the organic solvent are emulsified or dispersed in an aqueous medium in which, fine resin particles are dispersed (refer to PTL 9). In this proposal, as a coloring-agent dispersing resin, used is a poorly soluble polyester having an amide bond structure the weight-average molecular weight (Mw) of which is 5000 or more but 50,000 or less. Further, as a binding resin, the toner contains crystalline polyester which is poorly soluble in an organic solvent. Nevertheless, it is desired to further improve an image gloss level.